May 21 (Bloomberg) -- Pfizer Inc.'s stop-smoking pill Chantix has been linked to more than 3,000 reports in the U.S. of serious side effects, including suicides, heart trouble and aggression, a study said.

U.S. regulators should tighten warnings on the pill, prescribed 3.5 million times in the U.S. since it was approved in 2006, the nonprofit Institute for Safe Medication Practices said in its report. The group analyzed side effects reported to the Food and Drug Administration. The Federal Aviation Administration today banned the use of Chantix by pilots because of the report, spokesman Les Dorr said.

The FDA received more accounts of serious side effects tied to Chantix in the fourth quarter than from any other drug, the analysis found. In January, the agency said the drug may cause suicidal behavior and mood changes. The warnings need to say the medication shouldn't be used while driving, piloting airplanes or operating machinery because it may cause unconsciousness, seizures and hallucinations, the report said.

``These data provide a strong signal that the risks'' of Chantix ``have been underestimated, and show that a wide spectrum of serious injuries are being reported in large numbers,'' the Horsham, Pennsylvania-based Institute said in its report.

Pfizer, based in New York, fell 1.4 percent to $19.73 in extended trading after the close of the New York Stock Exchange, after dropping 27 percent in the previous 12 months.

Falling Sales

Prescriptions for Chantix in the U.S. have fallen by almost one-third since the FDA's warning in January as doctors start patients on other smoking aids, such as the nicotine patch or gum.

The new report prompted Sanford C. Bernstein & Co. analyst Tim Anderson to cut his sales estimates for the drug by 42 percent, to $758 million from $1.31 billion, in 2009 and by 60 percent, to $720 million from $1.82 billion, in 2015.

``The launch of Chantix had been a bright spot for Pfizer until safety issues surfaced in November 2007 that have since sent performance into a tailspin,'' said Anderson in a note to clients.

The FDA's adverse event reporting system is voluntary and there are no controls to ensure the side effects aren't coming from another drug, according to FDA spokeswoman Susan Cruzan. She said the agency is aware of the reports, though it hasn't been able to do a detailed investigation into all of them because of lack of resources. She couldn't immediately confirm the numbers used in the study.

More Research

More research is needed to make a definitive conclusion as to whether the drug, known chemically as varenicline, is responsible and why, ISMP said.

Pfizer said it saw a spike in Chantix reports after Carter Albrecht, a keyboard player for Edie Brickell and the New Bohemians, was killed while on the drug Sept. 5. It isn't uncommon for reports to increase following media attention of a drug, Pfizer said.

The company is investigating the reports, and the information it has so far is reflected in the drug's prescribing information. Pfizer is counting on Chantix, with $883 million in sales last year, to help replace some of the $12 billion in revenue it will lose in two years when the patent expires on its cholesterol pill Lipitor.

Unmet Need

``Chantix is a new medication that meets a huge unmet medical need,'' said Martina Flammer, senior medical director for Chantix, in a telephone interview today. ``It has the highest efficacy currently available and the important thing is to put this in context.''

The potential side effects may be a result of how Chantix works in the brain. The drug blocks the release of dopamine from nicotine receptors in the brain, taking away some of the pleasure of smoking. Dopamine also plays a role in regulating mood and movement.

Among the psychological side effects reported to the FDA were 28 suicides, 41 cases of homicidal thinking, 224 reports of heart trouble, 525 reports of hostility or aggression, and 397 cases of possible psychosis. There were also 173 serious injuries, including traffic accidents often associated with unconsciousness, dizziness, muscle spasms, or mental confusion.

There were also reports of severe skin conditions and new cases of diabetes in patients.

To contact the reporter on this story: Shannon Pettypiece in New York at spettypiece@bloomberg.net

Last Updated: May 21, 2008 18:44 EDT